International Journal of Scientific & Engineering Research, Volume 7, Issue 2, February-2016 636

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A Review of Casting Quality Problems and Potential Solutions in Small Scale Foundries

Lungu C and Paterson A E

Abstract — In this research hypothesis paper, a review of challenges encountered during production of castings in small scale under-

resourced foundries is presented. Small scale enterprises form the core of economies in many countries. However, castings produced in

these foundries have poor microstructure with unacceptable porosity in general. Important bearing properties of friction and wear

resistance are dependant on a good microstructure. Results from testing randomly obtained cast bearing samples showed irregular

microstructure, poor mechanical properties and high porosity.

Index Terms— Sustainable, quality castings, bronze sleeve bearings, design of process, microstructure, mechanical properties.

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1 INTRODUCTION

HIS paper reviews the challenges encountered in producing

bronze sleeve bearings successfully in small scale foundries

(SSFs) in general. Sleeve bearings, as opposed to rolling ele-

ment bearings, are commonly used in industrial equipment in open

cast mining and mineral processing equipment. This paper was moti-

vated by a research hypothesis that reliable short run production of

commercial quality bearings is sustainably achievable in small

foundries in Zambia at a cost that represents value for money.The

paper therefore explores common casting quality problems encoun-

tered in small foundry facilities and possible solutions that may be

applied to address them.

Fig. 1.1: Scrap flange sleeve bearing used as raw material at

Fox Foundry, Kitwe (February, 2014)

2 LITERATURE REVIEW

2.1 Quality bearings are castable in small scale foundries

Key variables such as composition, orientation of casting, mode

of molten metal pouring and the rate of cooling impact greatly on

the product microstructure as in [1], [2]. Employing counter gravi-

ty feeding of castings for example makes it possible to deliver mol-

ten metal under laminar conditions and avoids creation of bi-films

[2]. Bi-films are known to be sources of many types of defects in

castings such as porosity and cracks.

Getting casting parameters right and maintaining them through

out the casting process is critical to optimum product microstructure

and mechanical properties. SSFs face operational difficulties in set-

ting and monitoring casting quality parameters throughout the pro-

cess. Charge and melt compositions are important to measure regu-

larly just as process temperature is. It was observed at one foundry in

Kitwe, Zambia that temperature for example was only determined

prior to charging to the furnace. Temperature of the molten metal

itself was not measured at the Kitwe foundry. Installing and main-

taining reliable equipment for quality control is expensive. In addi-

tion to quality control equipment is the issue of documenting opera-

tional procedures which require highly skilled personnel to write and

implement.

Fig.2.1: Melting of bronze in progress at Fox Foundry, Kitwe

(February, 2014)

Although there are a handful of material combinations for bearing

applications as in [3], the use of copper base alloyed with tin and

nickel is widespread. By its nature bronze is a semi precious material

and is expensive and because of its high monetary value, security of

materials and operational cash flow are a concern for the SSF.

One possibile way of overcoming material security and cash

flow problems is getting production facilities for an SSF bronze pro-

ducer located in a major consumer’s premises. That would also be

referred to as a toll treatment arrangement. Bearing manufacture by

T

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the toll treatment route described above is a form of partnering with

major equipment suppliers or users and is also referred to in section

2.2 below. Lead as an alloying element has a dual role in bearings; it

acts as a lubricant and aids machining during finishing operations.

The amount of lead added to bronze alloys depends upon application

of the bearing and the level of machining required after casting. Typ-

ical amounts are from 1 to 3% but can be higher for high load - slow

speed bearing applications.

However, in recent years there has been a trend to eliminate the

use of lead in bronze bearings and other copper alloys in general and

substitute it with bismuth for environmental reasons [4]. The man-

agement of bismuth use however would be more difficult than that of

lead despite the environmental benefits that are possible from lead

substitution according to [4]. The environmental benefits of lead

substitution should therefore be weighed against foundry control

mechanisms possible for bismuth especially for a small foundry set

up.

Fig.2.2: Mould prepared for a secondary crusher counter-

shaft bearing billet at Fox Foundry (February, 2014)

On the other hand lubrication properties for bismuth are known to

be lower than those of lead and it tends to alloy with other bearing

constituents such as tin forming entirely new phases and introducing

new parameters to deal with. Final properties of bearings would re-

quire even stricter control under poorly resourced SSF conditions to

ensure good performance. Bismuth tends to expand more than lead

does at all treatment temperatures and can therefore lead to distor-

tions in the final castings.

2.2 Good attributes of small businesses

There is ample evidence that a cocktail of key success factors ex-

ist for long term survival of small businesses as stated in [5], [6], [7].

Individual businesses on their own find it difficult to deal effectively

with the aspect of production cost in a local context for example [6].

The local market for SSF output must generate value for money for

the producer to remain relevant in their market sector. Other than

production cost, the value equation includes delivery and perfor-

mance reliability of castings as well as production time savings be-

ing part of the major considerations.

The role played by central or local government in supporting

small businesses in terms of both soft and hard infrastructure was

also critical as elaborated in [7]. Timely government intervention in

the right areas and at the right times were known to greatly boost the

performance of the SSF.

A factor often ignored but which should be embraced by many

small businesses is that of association or partnership with a principal

original equipment manufacturer (OEM) as explained by [8].

A linkage between an SSF and OEM is beneficial in many ways and

helps to diffuse and transfer technology to the former entity where a

status of certified local supplier is attained.

Transformation planning and implementation in the face of exter-

nal shocks was more likely to succeed in small businesses than it

would in their larger counterparts as concluded by [7]. The model

used in [7] involved subjecting collected data to multiple regression

analysis. Factors such as firm size, age of firm, skilled workers and

export sales strongly influenced success on their own as individual

factors and acting together as collective factors. Small businesses can

adopt and use positive change management as described in transfor-

mation planning to improve on their performance and gain competi-

tiveness.

2.3 Design and implementation of production

A number of researchers have delved into the issue of the use of

expert systems (algorithms and proprietary software) to optimize

materials selection and production [9], [10], [11], [12]. In some cases

potential suppliers of input materials to the production process are

considered to be part of the equation. Process optimisation eliminates

costly trial and error techniques.

The software referred to above comes in many forms such as

Computer Aided Casting Process Selection (CACPS) and Materials

Process Advisor (MPA) to mention only but two. The Fuzzy Logic

approach has also been used [10], [11] in arriving at the appropriate

choice of materials and the production process particularly in situa-

tions where primary input data is subjective or imprecise. Proprietary

software is ordinarily expensive and out of reach of small foundries.

It must be stated that small foundries can make use of open source

versions of optimization software when available.

Although fairly old now, the Taguchi method of production

design (or design of experiment where appropriate) using orthogonal

arrays is referred to by many authors [13]. The first step involves

careful and prior analysis of the independent (input factors or xs) and

dependent variables (output factors, responses or ys) involved in a

given situation. The levels of each factor to be tested will then be

assigned and that would then help determine the total number of

experiments, n, to be conducted in one trial run and that is denoted as

Ln. The number of experiments actually required to optimize parame-

ters in the Taguchi approach is many times lower than that would be

possible in a conventional factorial design of the form pk, where p is

the number of levels to vary each x factor and k the total number of x

factors considered.

The Taguchi approach and other production or experiment opti-

mization techniques once applied diligently can result in significant

savings in process costs for small foundries. From the foregoing, it is

important even for the least resourced SSF to apply some form of

process optimization that is easy to understand and use. Optimiza-

tion ultimately saves time and a lot of resources for that matter and

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gives higher reliabilities for product performance.

3 CONCLUSIONS

3.1 Bronze sleeve bearings can be successfully produced in an un-

der-resourced small scale foundry just as they are in larger and better

equipped foundries providing parameters that impact on final cast

microstructure are optimised. Optimization can be achieved by ap-

plication of one of several techniques available commercially or just

adapted to suit a specific foundry environment.

3.2 Some of the important parameters to consider are cast tempera-

ture, melt composition, duration of pouring, weight of casting, rate of

cooling and mode of feeding. These factors individually or in combi-

nation affect the cast microstructure and consequently all other sec-

ondary cast properties.

3.3 A working partnership between a small foundry bronze producer

and a major consumer through a tolling arrangement eliminates un-

necessary costs and improves security of process materials for the

business and improves competitiveness.

3.4 Although lead remains the best alloying element in bronzes for

lubrication and machining purposes consideration should be given

for its substitution with bismuth where that is possible. However

whenever lead, it should be kept to the bearest required minimum.

3.5 Government role at all levels in terms of provision of both

hard and soft infrastructure is critical for the performance of the

small scale foundry bronze producer. Small businesses are also

quicker and more successful in adapting to change than their

larger counterparts.

3.6 The Taguchi approach on DOE remains the best option among

others even for the SSF set-up. It optimizes the use of resources and

places control of production parameters only on those that have the

greatest impact on the desired outcome.

ACKNOWLEDGMENT

The authors wish to thank the University of Zambia pro-gramme of Strengthening of Teaching Research and Consul-tancy Capacity (STRECC) for financial support. The University of the Witwatersrand for providing facilities and research advice during the course of work.

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Chozi Lungu is currently pursuing a doctor of philosophy programme in casting of bronze sleeve bearings at the School of Chemical and Metallurgi-cal Engineering, University of Witwatersrand, South Africa, PH-+27736334815. E-mail: chozi.lungu”students.wits.ac.za

Dr Tony Paterson is currently a senior Lecturer at the School of Chemical and Metallurgical Engineering,University of Witwatersrand in South Af-rica, PH-+27826024517. E-mail: tony.paterson@wits.ac.za

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